Since their early development, radar systems have detected and measured the distance to objects lying in their beams by transmitting high-power pulsed waveforms. An alternative way for measuring the distance to a reflecting object is the FMCW radar principle, which is based on continuous wave frequency chirps. For automotive radar systems this principle is very common because a high dynamic range can be achieved with respect to the frequency regulations and available MMICs (Monolithic Microwave Integrated Circuits).
FMCW radars operate by generating a continuous, i.e., not pulsed, transmitted signal using an oscillator that produces a linear frequency ramp. Thus, FMCW radars are also “chirped.” The linear frequency ramp is periodically generated, generally with a rapid linear transition from an end frequency to the initial frequency, i.e., the frequency ramp is produced with a sawtooth waveform. An object with sufficient radar cross-section lying within a beam of an FMCW radar antenna reflects a signal that is received by the radar system after a time delay proportional to the distance between the radar antenna and the reflecting object. In view of the substantially constant and known waveform propagation speed in a typical radar application, such distance measurements can potentially be made very accurately.
An FMCW distance measurement relies on the instantaneous frequency of the reflected signal being displaced from the instantaneous transmitted signal by a constant frequency difference dependent on the object's distance. Such constant frequency difference between transmitted and received signals relies on the linearity of the frequency ramp produced by the radar's oscillator. By multiplying a signal representing the transmitted signal by a signal representing the received signal, a waveform with a beat frequency is produced with a frequency proportional to the distance between the radar antenna and the reflecting object, with a small correction to account for Doppler shift resulting from the speed of the object relative to the radar antenna. By sampling the beat frequency waveform and estimating the beat frequency, a distance estimate can be produced.
VCOs (“voltage-controlled oscillators”) can produce an output signal with a frequency that is roughly linearly dependent on an input control signal. The lack of perfect linearity in the frequency control process corrupts the distance estimate. Two areas of concern arise: The first is the deviation of the estimated object distance from proportionality to the beat frequency of the multiplied signals. The second is the multiplicity of beat frequency components produced by nonlinearity of the VCO control process, which can potentially produce multiple object images for a single reflecting object.
A known technique for generating a periodic VCO control signal that produces a perfect VCO frequency ramp is to measure the deviation of VCO frequency from a perfect frequency ramp, and adjust the periodic VCO control signal for the next transmit cycle accordingly. Although this technique is simple in principle, its execution in the prior art has resulted in inaccurate ramp generation and/or complex and expensive circuit designs.
Thus, a challenge in designing an FMCW radar to generate a unique and accurate distance measurement for a reflecting object is generating a periodic control signal for a VCO to produce an output signal with a periodic, linear frequency ramp, wherein the VCO control signal is compensated for nonlinearities of the VCO.